There is a bug in WWW::Mechanize::Firefox 0.59:

Line 3327 is

        if( isNaN( target_rect.scalex * target_rect.scaley )) {
[download]

... and should be

        if( isNaN( target_rect.scalex * target_rect.scaley ) || target
+_rect.scalex * target_rect.scaley == 0) {
[download]

Version 0.60 will fix this - it should be on CPAN within the next few hours.

Update: But that bug does not "fix" your desired behaviour. There is no way to take a "complete screenshot" of that page with Javascript enabled, because that page resizes only a part of itself. If that page had an "outer" scrollbar, taking a screenshot would take a screenshot of the whole page, not only of the visible part. As it is, you will have to use the technique already mentioned by Anonymous Monk, take a screenshot of an element, and/or scroll a bit.